While signal exchanges within telecommunications networks and data communications networks have traditionally been accomplished by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. A single switching element 10 is shown in FIGS. 1 and 2. Waveguides are fabricated by depositing a lower cladding layer, a core, and an upper cladding layer on a substrate 12. The switching element is shown as including first and second input waveguides 14 and 16 and first and second output waveguides 18 and 20. The core material is primarily silicon dioxide, but with other materials that affect the refractive index of the core. The cladding layers are formed of a material having a refractive index that is substantially lower than that of the core material, so that optical signals are guided along the core material.
A trench 22 is etched through the core material to the silicon substrate in which the cladding layers and core material are formed. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the trench is filled with a vapor or gas. One wall of the trench 22 intersects the crosspoints of the waveguides 14-20. Thus, TIR diverts light from the first input waveguide 14 to the second output waveguide 20, unless an index-matching fluid is located within the gap between the first input waveguide 14 and the first output waveguide 18. The fluid within the trench has a refractive index that substantially matches the refractive index of the core material. An acceptable liquid is a combination of isopropyl alcohol and glycerol. Another acceptable liquid is M-pyrol.
In the embodiment of FIGS. 1 and 2, two microheaters 24 and 26 control the position of a bubble 28 within the fluid-containing trench 22. In the operation of the switching element 10, one of the microheaters is brought to a temperature sufficiently high to form the gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the microheater. In FIG. 1, the bubble is located at the intersection of the core waveguides 14-20. Consequently, an input signal along the first input waveguide 14 will encounter a refractive index mismatch upon reaching the wall of the trench 22. TIR causes the input signals to be diverted to the second output waveguide 20. Thus, the switching element is shown in a reflecting state in FIG. 1. The activation of the microheater 24 pins the bubble at the intersection, so that the reflecting state is maintained as long as the microheater is activated.
In FIG. 2, the microheater 24 at the intersection of the waveguides 14-20 has been deactivated and the second microheater 26 has been activated. The bubble 28 is strongly attracted to the activated microheater. This allows index-matching fluid to fill the gap at the intersection of the waveguides. The switching element is in a transmitting state, since the first input waveguide 14 is optically coupled to the collinear first output waveguide 18. Moreover, the second input waveguide 16 is optically coupled to the collinear second output waveguide 20.
FIGS. 1 and 2 represent only one available approach to manipulating fluid within a trench of a switching element. Other approaches are described in the Fouquet et al. patent. For example, a single heater may be used to vaporize index-matching fluid at the intersections of waveguides in order to toggle a switching element from a reflective state to a transmissive state.
The testing of a switching matrix which utilizes bubble manipulation to control signal paths has yielded very positive results. However, testing for long-term reliability (e.g., 25-year operation) has not been completed, particularly for large scale switching matrices. Consequently, there are still some concerns regarding the bubble-manipulation approach for directing signals in a telecommunications or data communications network. Other types of optical switches are commercially available, but suffer from one or more of cost efficiency, unwieldy size, poor performance, or a known lack of long-term reliability.
What is needed is an optical switching element and a method for fabricating switching matrices that enable optical switching with low insertion loss, low crosstalk, and high scalability, with long-term reliability.